If the fertilized eggs were being incubated by a broody hen in winter, would they survive through the cold? Because when she gets off the eggs to eat and drink, they will drastically cool down, will this kill the egg? How cold can they withstand? I want to incubate with a broody but the potential hen(s) won’t be mature enough until fall/winter.

I'm no help, only my older hens ever went broody but i don't think that's always the case. They had chicks in the winter occasionally but didn't raise many to adulthood iirc. It's adorable but they are very nervous around other birds and need peace and quiet. Hard enough to keep various groups thawed in the winter let alone segregating hens. This is my years ago experience, somewhat limited.

Your broody will know when she will be able to raise chicks safely. She is not likely to sit in the dead of winter, however spring cometh so perhaps your broody will oblige you. Patience, my friend, patience.....

Well, I’ve heard of hens raising chicks/becoming broody in winter. So I’m really just asking if the eggs won’t die from drastic temperature changes when she gets off of them.

(The chickens would be mature enough around December or January. But it all depends on if we want them in June or July, so I’m not that sure if it’ll be that cold then. It’s just so unpredictable with this winter throwing me off guard.)